Electroluminescent device



Apr ilx5, 1960 T. JAY, JR 2,931,915

ELECTROLUMINESCENT DEVICE Filed Feb. 25, 1957 TRANSPARENT /0 CONDUCT/V5 FILM 2 ca/vsrA/vr amscr Q HOMOGEA/EOUS VOL TA GE ELECTROLUMl/VESCEWT L A YER PH0T76'0/VDUC77VE [N LAYER Z4- TRANSPARENT ZMPL/ Tl/PE' CO/VPUC TI VE MODULATED FILM V0 1- TA 65' /3 HOMom-wtous n. 5: mm (/MINESC 51v 1' L A YER i 30 TRANSPARENT 22 nvsummvc FILM 32 TRANSPARENT INSULATING FILM INVENTOR ELECTROLUMINESCENT DEVICE Theodore Jay, In, Tenafly, N.J., assignor, by mesne assign'ments, to Sylvania Electric Products Inc., Wilmington, Del., a corporation of Delaware Application February 25', 1957, Serial No. 641,981 Clainis'. 611250-213 can be used to transform electrical energy to light energy.

Phosphors of this type are said to be electroluminescent. A first type of electroluminescent layer is formed solely from electroluminescent phosphor and is termed a homogeneous layer. A second type of electroluminescent layer is formed from a suspension of electroluminescent powders in dielectric media, as describedfor example inthe' coperiding, patent application Serial No. 306,909, filed August 28,- 1952, by Norman L. Harvey. M

conventionally an electroluminescent layer of either type is interposed and electrically connected between first and second transparent electrically conductive films, thus forming an electroluminescent cell. A voltage isapplied between the two'films, and the cell luminesces in accordance with the magnitude of the applied voltage. When the first type of layer is used, a constant or variable voltage can be applied across the cell; when the second layer is used, the applied voltage must be a variable voltage.

To increase the light output of such a cell, it has been proposed to modify the cell in such manner as to render thecellresponsive to a modulated electromagnetic wave as; for example, modulated light. In other words, the cell ismodified to act as a light amplifier.

In one modification of this type, a photoconductive layer is interposed between the electroluminescent layer andoneof the films. The resulting cell thus contains electroluminescent and photocondl'ictive layers electrically connected together in series.

In thisjrnodified cell, the electrical characteristics of thephotoconductive and electroluminescent layers; are chosen such that the dark resistance of the photoconductive layer would be high relative to the resistance of the electroluminescent layer. Therefore, when avoltage. is applied across the series combination of the two layers, and the photoconductive layeris notilluminated, the resultant potential drop across the photoconductive ele'-' Ii have invented. alight amplifier of this type which re -1' spgfidsto ani electrical signal rather than-to modulated light.

ice i Accordingly, it is an object of the'present invention to provide a new and improved light amplifier of the character indicated.

Another object is to modify an electroluminescent cell containing a photoconductive layer in such manner that the modified cell responds to anelectrical signal rather than modulated light.

Still another object is to improve light amplifiers through the action of a second electroluminescent layer.

Yet another object is to provide a new and improved electroluminescent cell containing first and second elec troluminescent layers and a photoconductive layer.

These and other objects of my invention will either be explained or will become apparent hereinafter.

In accordance with the principles of my invention, 1 provide an electroluminescent amplifier comprising a homogeneous electroluminescent layer, a heterogeneouselectroluminescent layer, a photoconductive layer, and first, second and third electrically conductive films. The first film is in electrical contact with one side of the homogeneous layer; the second film is in electrical .contact with one side of the photoconductive layer and one side of the heterogeneous layer; the third film is in elec-' trical contact with the other side of the heterogeneous layer. The other sides of the homogeneous layer and the photoconductive layer are in electrical contact with" each other.

A direct voltage is applied between the first and second films, and an amplitude varying electrical signal is applied between the second and third films. As a consequence, the heterogeneous layer is energized, its lightoutput varying in accordance with the amplitude variations of the applied signal. When the heterogeneous layer does not emit light, the voltage drop across the homogeneous and photoconduc: tive layers produced by the direct voltage appears primarily across the photoconductive layer. and the homogeneouslayer does not luminesce appreciably.

When the heterogeneous layer does emit light under the influence of the applied signal, this emittedlight will irradiate the photoconductive layer, thus reducing its resistance. This reduction in resistance decreases the voltage drop across the photoconductive layer and, consequently, increases the voltage drop across the homogeneous layer to a point at which the homogeneous layer will emit light. The light output from the homogeneous layer varies with the light output from the heterogeneouslayer and thus varies in accordance with the applied signal. 7

In a sense, the portion of the structure interposed be tween the first and second conductive films can be re: garded as analogous to an anode-cathode circuit of a vacuum tube amplifier, while the portion of the structure interposed between the second and third conductive films can be regarded as analogous to the grid-cathode circuit for this amplifier. p u

The light output of the amplifier thus far. described, while acceptable for applications requiring only moderate amounts of light, is insufi'icien t for many applications;

Moreparticularly, in this amplifier the dark'resistance;

of the photoconductive layer is. dependent upon the thickness of this layer; sincejthe dark resistance must be high relative to the resistancecfthe homogeno' electroluminescent layer, the photoconductive layerm st be relatively thick. Howeverpin thistype'of amplifier, 1 most of the light incident upon the surface of the photo* conductive layer isabsorbed in the surface itself. With the layer thickness required, the percentage change 'in In one embodiment of myzinvention the light output of the amplifier is increased by matching the homogeneous and photoconductive layers in the manner taught in the. copending patent application of G. Rothschild, filed November I, 1956, Serial No. 619,729, now Patent No. 2,915,641; i.e. the homogeneous layer has a wavelength dependent light emission characteristic at which the emission attains a maximum value for'a given range of wavelengths and the photoconductive layer has a wavelength dependent photoconductive sensitivity characteristic at which the sensitivity of the layer as a whole attains a maximum value over the same given range of wavelengths. In this arrangement, as explained in more detail in the aforementioned application Serial No. 619,729, thedark resistance of the photoconductive layer can be made high relative to the resistance of the electroluminescent layer by appropriate adjustment of the thickness of the photoconductive layer. Further, when this relationship of the light emission characteristics of the electroluminescent layer to the photoconductive sensitivity characteristic of the photoconductive layeris established, the photoconductive action is not confined to the surface of the photoconductive layer, but rather'extends substantially throughout the entire layer. In a second embodiment of my invention, a portion of the photoconductive layer is insulated both from the homogeneous layer and from the second film in the manner taught in the copending patent application of Livingston, filed July 2, 1956, Serial No. 595,297, now Patent No. 2,874,308; i.e. a major portion of the photoconductive layer is electrically insulated both from the homogeneous layer and from the second conductive film in such manner that the end to end resistance along the'surface of the photoconductive layer is electrically connected in series with the homogeneous layer and the second conductive film. The photoconductive layer can be extremely thin, thus permitting the entire layer to be irradiated with light emitted from the heterogeneous electroluminescent layer. Further, since the resistance of the photoconductive layer is determined by the end to end separation along its surface, its dark resistance can be made as high as desired.

Illustrative embodiments of my invention will now be described in detail with reference to the accompanying drawings wherein:

Fig. 1 is a cross sectional view of an embodiment of my invention; and

Fig. 2 is a cross sectional view of another embodiment of my invention. 7

Referring now to Fig. 1, there is provided a sandwichlike structure comprising in the order named a first electrically conductive transparent film 10, a first homogeneous electroluminescent layer 12, a photoconductive layer 14, a second transparent electrically conductive film 16, a second heterogeneous electroluminescent layer 18 and a third electrically conductive film 20. A first pair of terminals 22 are connected between conductive films 10 and 16 and a second set of terminals 24 are minals 24. This signal energizes the second electro-- luminescent layer 18, the magnitude of the luminescence thus produced being dependent upon the amplitude of the signal. The light emitted from this layer 18 irradiates the photoconductive layer, thus reducing its resistance and causing the first electroluminescent layer to luminesce. The light output from the first electroluminescent layer 12'will vary in accordance with the modulated signal, but by virtue of the relative high differcnce in potential across the 'layer 12 will beat a higher level than the light output from the second electroluminescent layer 18. Hence, this structure acts as a light amplifier.

In order to increase the light output from such a. structure, the electrical characteristics of layers 12 and 14 can be matched in the manner indicated previously and described in more detail in the aforementioned copending patent application Serial No. 619,729.

Fig. 2 shows a modification of Fig. 1 in which the electrical interconnection between the photoconductive layer 14 and the electroluminescent layer 12 is performed in the manner previously described and outlined in more detail in the copending patent application Serial No. 595,297. As before, the structure contains first, second and third electrically conductive films 10, 16, and 20 and first and second electroluminescent layers 12 and 18 and a photoconductive layer 14. However, a major portion of the homogeneous electroluminescent layer 12 is insulated separately from a corresponding portion of the photoconductive layer 14 by a first transparent insulating film 30. In a similar manner, a major portion of the second electroluminescent layer 18 is separated from a corresponding portion of the photoconductive layer 14 by a second transparent insulating film 32. An end portion 34 of the photoconductive layer 14 is in electrical contact with the first electroluminescent layer 12 and an opposite end portion 36 of the photo conductive layer 14 is in electrical contact with the second electroluminescent layer 18.

. Thus, in this arrangement, the end to end resistance of the photoconductive layer along that portion of the surface extending between ends 34 and 36 of the phot0- conductive layer is electrically connected in series with both the first and second electroluminescent layers. This device is electrically controlled by means of a direct voltage and a modulated signal in the same manner as in Fig. 1.

. While I have shown and pointed out my invention as applied above, it will be apparent to those skilled in the art that many modifications can be made within the scope and sphere of my invention as defined in the claims which follow.

. What is claimed is:

1. An electroluminescent light amplifier comprising a homogeneous electroluminescent layer; a photoconduc-- tive layer, one side of said photoconductive layer being in electrical contact with one side of said homogeneous layer; a heterogeneous electroluminescent layer; and first, second, and third electrically conductive films, said first film being in electrical contact with the other side of said homogeneous layer, said second film being in electrical contact with theother side of said photoconductive layer and one side of said heterogeneous layer, said third film being in electrical contact with the other side of said heterogeneous layer.

2. An electroluminescent amplifier as set forth in claim 1, further including a first pair of terminals respectively connected to said first and second films and a second pair of terminals respectively connected to said second and third films.

3. An electroluminescent amplifier asset forth in claim V homogeneous electroluminescent layer; a photoconductive layer, one side of said photoconductive layer being in electrical contact with one side of said homogeneous layer;'a heterogeneous electroluminescent layer; and first, second and third electrically conductive films, said first film being in electrical contact with the other side of said homogeneous layer, said second film being, in electrical, contact with the other side of said photoconductive layer' and one'side of said heterogeneous layer, said third film being in electrical contact with the other side of said heterogeneous layer, said homogeneous layer having a wavelength dependent light' emission charactcristic'at which the emission attains a maximum value for a given range of wavelengths, said photoconductive layer having a wavelength dependent photoconductive sensitivity characteristic at which the sensitivity attains a maximum value for the same given range of Wavelengths.

5. An electroluminescent light amplifier comprising a homogeneous electroluminescent layer; a heterogeneous electroluminescent layer; a photoconductive layer; and first, second and third electrically conductive films, said first and third films being in respective electrical contact with one side of said homogeneous layer and said heterogeneous layer, said second film being in electrical contact with the other side of said heterogeneous layer and with one side of said photoconductive layer, the other side of said homogeneous layer being in electrical contact with the other side of said photoconductive layer; means to apply a constant direct voltage between said first and second films; and means to apply an amplitude modulated signal between said second and third films.

6. An electroluminescent amplifier as set forth in claim 5, wherein said second film is a common ground for both said voltage and said signal.

7. An electroluminescent light amplifier comprising a homogeneous electroluminescent layer; a heterogeneous electroluminescent layer; a photoconductive layer; first, second and third electrically conductive films, said first and third films being in respective electrical contact with one side of said homogeneous layer and said heterogeneous layer, said second film being in electrical contact with the other side of said heterogeneous layer and with one side of said photoconductive layer, the other side of said homogeneous layer being in electrical contact with the other side of said photoconductive layer; means coupled between said first and second films to condition said homogeneous layer almost to the point of excitation in the absence of light excitation of said heterogeneous layer;

and means coupled between said second and third films to' supply an amplitude modulated signal to said heterogeneous layer and thereby produce light excitation thereof.

8. An electroluminescent light amplifier comprising a homogeneous electroluminescent layer; a heterogeneous electroluminescent layer; a photoconductive layer; and first, second and third electrically conductive films, said first and third films being in respective electrical contact with one side of said homogeneous layer and said heterogeneous layer, said second film being in electrical contact with the other side of said heterogeneous layer and with one side of said photoconductive layer, the other side of said homogeneous layer being in electrical contact with the other side of said photoconductive layer; all but one end portion of said other side of said photoconductive layer being electrically insulated from said homogeneous layer, all but the opposite end portion of said one side of said photoconductive layer being electrically insulated from said second film.

9. An electroluminescent light amplifier comprising a homogeneous electroluminescent layer; a heterogeneous electroluminescent layer; a photoconductive layer; first, second and third electrically conductive films, said first and second films being transparent, said first and third film being in respective electrical contact with one side of said homogeneous layer and said heterogeneous layer, said second film being in electrical contact with the other side of said heterogeneous layer and with one side of said photoconductive layer, the other side of said homogeneous layer being in electrical contact with the other side of said photoconductive layer; first means to electrically insulate all but one end portion of said other side of said photoconductive layer from said homogeneous layer, and second means to electrically insulate all but the opposite end portion of said one side of said homogeneous layer.

10. An electroluminescent amplifier as set forth in claim 9, wherein said first and second means respectively are first and second transparent insulating films.

References Cited in the file of this patent UNITED STATES PATENTS 2,594,740 De Forest et a1. Apr. 29, 1952 2,791,723 Nagy et a1 May 7, 1957 2,816,236 Rosen Dec. 10, 1957 2,835,822 Williams May 20, 1958 2,874,308 Livingston Feb. 17, 1959 2,915,641 Rothschild Dec. 1, 1959 OTHER REFERENCES A Solid-State Image intensifier, by Orthuber and Ullery, Journal of the Optical Society of America, volume 44, No. 4, April 1954. 

1. AN ELECTROLUMINESCENT LIGHT AMPLIFIER COMPRISING A HOMOGENEOUS ELECTROLUMINESCENT LAYER, A PHOTOCONDUCTIVE LAYER, ONE SIDE OF SAID PHOTOCONDUCTIVE LAYER BEING IN ELECTRICAL CONTACT WITH ONE SIDE OF SAID HOMOGENEOUS LAYER, A HETEROGENEOUS ELECTROLUMINESCENT LAYER, AND FIRST, SECOND, AND THIRD ELECTRICALLY CONDUCTIVE FILMS, SAID FIRST FILM BEING IN ELECTRICAL CONTACT WITH THE OTHER SIDE OF SAID HOMOGENEOUS LAYER, SAID SECOND FILM BEING IN ELECTRICAL CONTACT WITH THE OTHER SIDE OF SAID PHOTOCONDUCTIVE LAYER AND ONE SIDE OF SAID HETEROGENEOUS LAYER, SAID THIRD FILM BEING IN ELECTRICAL CONTACT WITH THE OTHER SIDE OF SAID HETEROGENEOUS LAYER. 